1. Field of the Invention
The present invention relates to microcapsules wherein the crosslinking density of the capsule wall can be controlled, particularly to microcapsules suitable as a thermosensitive recording material, and to a process for manufacturing the microcapsule.
2. Description of the Related Art
The thermosensitive recording materials that are now commonly used as recording media in facsimile machines and printers are mainly obtained by coating and drying a solid dispersion of an electron donor dye precursor on a support. Recording methods using such electron donor dye precursors have the advantage that the materials are easy to obtain, and offer a high coloring density and coloring speed. However, there were storage difficulties after recording. Due to adhesion as a result of heating or solvents, they tended to become discolored, and with the problems regarding storage properties and reliability of recorded images, many methods of improving them have been studied.
There is a proposal of a process for improving the storage of recorded images, in which the electron donor dye precursor in microcapsules is enclosed, and thereby the developer and dye precursor in the recording layer are separated. According to this process, enhanced color properties and image stability are obtained.
Examples of other thermosensitive recording materials are so-called diazo thermosensitive recording materials using diazonium salts. A diazonium salt reacts with a phenol derivative or a compound having an active methylene group (coupler) to form a dye (a basic compound is usually used as a reaction accelerator), and it also has photosensitivity and loses its activity by irradiation with light. Recently, this principle has been applied also to thermosensitive recording materials. An optical fixing thermosensitive recording material has been disclosed in which a diazonium salt and a coupler are reacted by heat to form an image, and the image is then fixed by light irradiation (Koji Sato et al. (1982) Institute of Image Electronics Engineers of Japan Vol. 11th, No. 4, pp. 290-296).
However, as thermosensitive recording materials using diazonium salts are highly chemically active, even at low temperature, there is a problem that the diazonium salt and coupler gradually react in storage, and shelf life is short. In one process for dealing with this, the diazonium salt is enclosed in a microcapsule to isolate it from the coupler, water and the basic compound (Tomomasa Usami et al. (1987) Society of Electrophotography of Japan, Vol. 26, Part 2, pp. 115-125).
Multicolor thermosensitive recording materials have attracted attention, as applying thermosensitive recording materials. In this case, reproduction of the multicolor image by thermosensitive recording is said to be difficult, compared with an electrophotographic recording method or an ink-jet method. Multicolor thermosensitive recording materials are obtained by laminating, on a support, two or more thermosensensitive colored layers which contain an electron donor dye precursor enclosed in microcapsules and a developer as main components, or a diazonium salt enclosed in microcapsules and a coupler which colors by reacting with the diazonium salt on heating. In these multicolor thermosensitive recording materials, in order to obtain outstanding color reproduction compared with a monochromatic thermosensitive recording material, it is required that the heat coloring properties of the microcapsules be rigorously controlled.
In the related art, to enclose an electron donor dye precursor or a diazonium salt compound in microcapsules, emulsion dispersion is generally used. In this enclosing procedure, these compounds are dissolved in an organic solvent (oil phase), and the compounds dissolved in the organic solvent, is then added to emulsion dispersion in an aqueous solution (aqueous phase) of a water-soluble polymer, so as to be subjected to emulsion dispersion. In this regard, a process for microcapsuling has been disclosed, in which the monomer or prepolymer used as a capsule wall of the microcapsule material is added to one of the organic solvent phase and the aqueous phase, so as to form a capsule wall of polymer at the interface of the organic solvent phase (refer to, for example, Asaji Kondo. Microcapsules. (1970) Nikkan Kogyo Shinbun Ltd.; and Kondo et al. Microcapsules (1977) Sankyo Publishing Co., Ltd.). Examples of the material for a capsule wall of a microcapsule are gelatine, alginate, cellulose, polyurea, polyurethane, melamine resin and polyamide (nylon). Of these, polyurea and polyurethane have a glass transition temperature (Tg) within the range of room temperature to 200° C., so the material for a capsule wall of a microcapsule obtained shows a heat response and they are suited to form thermosensitive recording materials.
In the process for manufacturing microcapsules having a capsule wall of polyurethane or polyurea, a diazonium salt or an electron donor dye precursor are first dissolved in an organic solvent, a multivalent isocyanate compound is added thereto, and the organic phase solution is emulsified in an aqueous solution of a water-soluble polymer. Thereafter, a catalyst is added to accelerate a polymerization reaction in the aqueous phase, or the temperature of the emulsion is raised to polymerize the multivalent isocyanate compound with a compound having an active hydrogen such as water, so as to form the capsule wall. Examples of the multivalent isocyanate compound forming the capsule wall of polyurea or polyurethane, are the adduct of 2,4-tolylene diisocyanate and trimethylol propane, the adduct of xylene diisocyanate and trimethylol propane, and the like (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 62-212190).
However, even in the case of a polyurea or polyurethane capsule wall using this multivalent isocyanate compound, the problem of the short shelf life when using the aforesaid diazonium salt has not been fully improved. Namely, in the case of a thermosensitive recording material whose shelf life is not sufficiently long, if it was stored for example under high temperature, high humidity conditions after manufacture until it was used, a background coloration known as “fogging” appears, and the visibility of the printed image declined. Therefore, it has been desired to further increase shelf life while maintaining high coloring properties.
In the aforesaid multicolor thermosensitive recording material, cyan, magenta and yellow thermosensitive recording layers are provided. Since these thermosensitive recording layers are printed at different heating temperatures, a superior heat response to that of the thermosensitive recording layer formed of an ordinary thermosensitive recording material is required. However, the polyurea or polyurethane capsule wall in the related art has not fully satisfied this demand.